Preparation of ω-hydroxynonanoic acid and its ester derivatives

Article abstract of DOI:10.1007/s11746-999-0070-y

Methyl ricinoleate was ozonized in methanol or in acetic acid and the intermediate hydroperoxides were reduced electrochemically on Pb-cathode to give 9-hydroxynonanoic acid 1 in high yields. The acid 1 was also prepared by direct castor oil ozonolysis in methanol followed by sodium borohydride reduction of the intermediate hydroperoxides. The cost of the electricity for the electroreduction was at least 30 times lower as compared with sodium borohydride consumption. 9-Hydroxynonanoic acid was then transformed to alkyl 9-acetoxynonanoates 3a-3d, for which ¹H nuclear magnetic reasonance, mass, and infrared spectra are given. Esterification of the hydroxy acid 1 with boric acid and pyrolysis of the resultant orthoborates produced 8-nonenoic acid 4 in a 45% yield. Reaction of 4 with lower aliphatic alcohols in presence of Amberlyst 15 produced alkyl 8-noneates 5a-5d along with some amounts of a cis/trans mixture of alkyl 7-noneates.

Full text of DOI:10.1007/s11746-999-0070-y

Preparation of ω-Hydroxynonanoic Acid
and Its Ester Derivatives
Jozef Kula*, Krzysztof Smigielski, Thuat B. Quang, Iwona Grzelak,
and Magdalena Sikora
Institute of General Food Chemistry, Technical University of Lodz, 90-924 Lodz, Poland
ABSTRACT: Methyl ricinoleate was ozonized in methanol or in acids since they can readily be converted to the corresponding
acetic acid and the intermediate hydroperoxides were reduced
electrochemically on Pb-cathode to give 9-hydroxynonanoic
acid 1 in high yields. The acid 1 was also prepared by direct
castor oil ozonolysis in methanol followed by sodium borohy-
dride reduction of the intermediate hydroperoxides. The cost of
the electricity for the electroreduction was at least 30 times
lower as compared with sodium borohydride consumption. 9-
Hydroxynonanoic acid was then transformed to alkyl 9-ace-
toxynonanoates 3a–3d, for which 1H nuclear magnetic reaso-
nance, mass, and infrared spectra are given. Esterification of the
hydroxy acid 1 with boric acid and pyrolysis of the resultant or-
alkynic compounds (9). 8-Nonenoic acid is a starting material
for a plant growth inhibitor synthesis (10) and is an intermedi-
ate in the synthesis of “royal jelly acid” of the honey bee (11).
9-Hydroxynonanoic acid can be manufactured by ozonol-
ysis of C9–C10 unsaturated fatty acids such as oleic, linoleic,
or ricinoleic acid followed by the reduction of the intermedi-
ate ozonides (12,13), or by the reduction of 9-oxo-nonanoic
acid with sodium borohydride (1).
Some time ago we reported short syntheses of (E)-2-nonenal
(14) and enantiomerically pure (R)-(+)-γ-decalactone (15) from
thoborates produced 8-nonenoic acid 4 in a 45% yield. Reac- castor oil. The aim of the present study was to economically
tion of 4 with lower aliphatic alcohols in presence of Amberlyst
15 produced alkyl 8-noneates 5a–5d along with some amounts
of a cis/trans mixture of alkyl 7-noneates.
synthesize 9-hydroxynonanoic acid from methyl ricinoleate or
directly from commercial castor oil, to prepare some diesters of
the hydroxy acid, and to transform ω-hydroxynonanoic acid
into 8-nonenoic acid and its esters.
Paper no. J9066 in JAOCS 76, 811–817 (July 1999).
KEY WORDS: Castor oil, electrochemical reduction of hy-
droperoxides, esters, 9-hydroxynonanoic acid, 8-nonenoic
acid, ozonolysis, sodium borohydride reduction.
EXPERIMENTAL PROCEDURES
Commercial castor oil [n20D = 1.475, [α]20D = +5.5° (neat)]
was used as starting material. Methyl ricinoleate [92% pure,
gas chromatography (GC) analysis] was prepared from castor
oil as described (16). The crude crystalline 9-hydrox-
ynonanoic acid was employed for further transformation.
Amberlyst 15 was purchased from Fluka Co. (Buchs,
Switzerland). GC determination of all products was per-
formed with Carlo Erba instrument (Milano, Italy) model
MEGA 5300 with a flame-ionization detector using a 30 m ×
0.32 mm DB-17 capillary column. The temperature was pro-
grammed from 60 to 300°C at 6°C/min. Nuclear magnetic
resonance (NMR) spectra were obtained with Bruker AC
spectrometer (Karlsruhe, Germany) operating at 250 MHz.
All spectra were recorded in CDCl3 solvent, and chemical
shifts were reported as ppm (δ) from tetramethylsilane
(TMS). GC-mass spectra (MS) were obtained with a Carlo
Erba GC 8000 coupled to MD 800 Fisons Instruments (Poole,
United Kingdom). The apparatus was equipped with the same
column as for GC analysis. Helium was used as carrier gas;
electron ionization 70 eV. Infrared (IR) spectra were recorded
with a Specord 71 spectrophotometer (Carl Zeiss, Jena, Ger-
many) in film. Electroreduction of intermediate hydroperox-
ides was carried out in a three-electrode electrolyzer (Scheme
ω-Hydroxyalkanoic acids and their esters have value as inter-
mediates in both laboratory and industrial use. 9-Hydrox-
ynonanoic acid is an intermediate in the synthesis of phos-
pholipids used as membrane model systems for investigating
biochemical interactions that occur within the biological mi-
lieu (1). On the other hand, some esters of the acid are em-
ployed for the preparation of synthetic antigens (2,3). More-
over, 9-hydroxynonanoic acid can be considered as a natural
product owing to its presence in royal jelly of the honey bee
(4,5) and in some phospholipid and lipopolysaccharide frac-
tions of agricultural soil (6). It is also a building block for the
naturally occurring antibiotic pseudomonic acid A, which has
high antimicrobial activity (7,8).
Aliphatic acids or esters with an alkenic bond at the end of
the chain remote from the carbonyl group, which can be consid-
ered as the ω-hydroxyalkanoic acid derivatives, are important
strategic compounds in the synthesis of polyunsaturated fatty
*To whom correspondence should be addressed at Institute of General Food
Chemistry, Technical University of Lodz, ul. Stefanowskiego 4/10, 90-924
Lodz, Poland. E-mail: jkula@snack.p.lodz.pl
Copyright © 1999 by AOCS Press
811
JAOCS, Vol. 76, no. 7 (1999)

812
J. KULA ET AL.
sodium bicarbonate in 100 mL of water was added and the
mixture was then purged with nitrogen and transferred to the
cathode cell. The electrolysis was carried out at 16–20°C until
8 F/mole had been delivered (current density 1.43 A/dm2).
Some 80 mL of methanol was evaporated in vacuum, and the
residue was extracted with ether (4 × 80 mL). The aqueous
solution was cooled to 0°C and made acidic with concentrated
hydrochloric acid (pH < 2). The liberated 9-hydroxynonanoic
acid was extracted with ether (4 × 50 mL) and the combined
extracts were washed with water until neutrality was achieved
and dried over anhydrous MgSO4. The solvent was removed
by rotary evaporation to give 9.4 g (92%) of crystalline prod-
uct, which was then recrystallized from ether to yield 9-hy-
droxynonanoic acid 1 of 98% purity, m.p. 51–52°C [literature
value (1), 53°C].
Methyl ricinoleate ozonolysis in acetic acid. A solution of
20 g (0.059 mole) of ricinoleic acid in acetic acid (200 mL)
was cooled to 7°C and oxygen containing 4.7 vol% of ozone
was bubbled through until a positive test with potassium io-
dide was reached. Then, a solution of 11.2 g (0.14 mole) of
sodium acetate in water (35 mL) and acetic acid (100 mL)
were added, and the whole was placed in the cathode cell. The
electrolysis was conducted at 19–26°C until 8 F/mole had
been delivered (current density 1.43 A/dm2). Excess of acetic
acid (ca. 180 mL) was evaporated in vacuum, and 120 mL of
water was added. The mixture was extracted with ethyl ace-
tate (3 × 100 mL). The combined extracts were washed to
neutrality with brine and dried over anhydrous Na2SO4. After
solvent evaporation the slurry (29 g) was treated with a solu-
tion of sodium hydroxide (4.8 g, 0.12 mole) in water (30 mL)
and ethanol (100 mL), and refluxed for 5 h. Ethanol was evap-
orated under reduced pressure, and the residue was extracted
with ether (3 × 50 mL). The inorganic layer was cooled to
0°C, treated with concentrated hydrochloric acid (pH < 2) and
extracted with ether (3 × 80 mL). The combined extracts were
washed to neutrality with water, dried over anhydrous
Na2SO4, and the solvent was evaporated. The crystalline
residue (8.9 g, 87%) was recrystallized from ether to give 9-
hydroxynonanoic acid 1 of 98% purity, m.p. 51–53°C.
Castor oil ozonolysis. A solution of 60 g castor oil (con-
taining ca. 0.16 mole of ricinoleic acid) in methanol (220 mL)
was cooled to –15°C and oxygen containing 4.7 vol% of
ozone was bubbled through until a positive test with potas-
sium iodide was reached. The mixture was then purged with
nitrogen, and 6.9 g (0.18 mole) sodium borohydride was
added portionwise, with agitation at a rate that kept the tem-
perature of the reaction mixture at –10 to 5°C (4 h). The stir-
ring was continued for another 2 h at room temperature. A so-
lution of 6.5 g (0.17 mole) sodium hydroxide in water (20
mL), was added and the mixture was refluxed for 2 h.
Methanol (ca. 190 mL) was removed by vacuum distillation,
and 120 mL water was added. 1,3-Nonandiol and other
nonacidic components were extracted with ether (3 × 120
mL), and the aqueous solution was cooled to 0°C and made
acidic (pH < 2) with concentrated hydrochloric acid at a rate
that kept the temperature below 5°C. The liberated 9-hydrox-
SCHEME 1
1). The cathode cell (200 × 50 × 30 mm) was made of ceramic
plate and placed in a thermostatic glass container of 1 L ca-
pacity. The cathode (175 × 40 × 1 mm) was sheet lead. Each
anode (150 × 45 × 1 mm) was sheet acid-proof steel. The
cathode was placed centrally between two anodes at a dis-
tance of 30 mm. Prior to use, the electrodes were cleaned me-
chanically and the Pb-cathode was additionally etched for 3
min with a 10% aqueous solution of HNO3 and rinsed with
distilled water. Anolytes were prepared from a 10% aque-
ous/organic solution of NaOH (67.1 g NaOH in 375 mL of
water and 375 mL of methanol) for electroreduction in
methanol, and from a solution of 82.03 g (1.0 mole) of
sodium acetate in 60.05 g (1.0 mole) acetic acid and 1000 mL
of water for electroreduction in acetic acid.
Methyl ricinoleate ozonolysis in methanol. A solution of
20 g (0.059 mole) of methyl ricinoleate in methanol (100 mL)
was cooled to –10°C, and oxygen containing 4.7 vol% of
ozone was bubbled through until a positive test with potas-
sium iodide was reached. A solution of 6.0 g (0.071 mole)
JAOCS, Vol. 76, no. 7 (1999)

ω-HYDROXYNONANOIC ACID AND ITS DERIVATIVES
813
tered off, and the solution was washed twice with sodium bi-
carbonate solution and water and concentrated to give crude
esters 5. Vacuum distillation of the residue furnished unsatu-
rated esters 5 of 77–96% purity (Table 2).
ynonanoic acid was extracted with ether (4 × 150 mL) and the
combined organic layers were then washed with water (3 ×
40 mL) and dried over anhydrous MgSO4. The solvent was
removed by rotary evaporation, and the crude solid (21 g,
75%) was recrystallized from ether to give 9-hydrox-
ynonanoic acid 1 of over 97% purity (m.p. 50–52°C).
RESULTS AND DISCUSSION
Alkyl 9-acetoxynonanoates 3a–3d. To a solution of 0.02
mole of alkyl 9-hydroxynonanoate 2 (prepared by direct esteri-
fication, see references 1,12,13) in freshly distilled acetic anhy-
dride (0.04 mole), 0.5 g anhydrous sodium acetate was added.
The mixture was stirred at 120°C for 2 h, and then 10 mL of
water was added and stirring was continued for another 30 min
at 50–60°C. The aqueous layer was extracted with ether (3 × 30
mL). The combined organic layers were then washed with a
concentrated sodium bicarbonate solution (5 × 20 mL) and
water, and dried over anhydrous MgSO4. After solvent evapo-
ration the crude diesters 3 were purified by distillation under re-
duced pressure to yield product of 94–99% purity (Table 1).
8-Nonenoic acid (4). A mixture of 13.5 g (0.077 mole) 9-hy-
droxynonanoic acid 1, 1.7 g (0.027 mole) boric acid, and 150
mL of chloroform was refluxed under azeotropic conditions
until ca. 1.4 mL water was collected. The solvent was removed
by vacuum evaporation and the residue, 13.8 g [1H NMR:
9.2–8.0 (br. s, 1H), 3.75 (t, J = 6.5 Hz, 2H), 2.35 (t, J = 7.0 Hz,
2H), 1.66–1.54 (m, 4H), 1.32 (m, 8H)] was heated, under argon,
up to 290–450°C to yield 8.8 g of crude product (b.p.
95–215°C). This material was fractionally distilled under argon
to give 5.5 g (45%) 8-nonenoic acid 4 of 95% purity (GC), b.p
98–101°C/0.5 mm Hg. Analysis: Found C 69.00, H 10.33%;
Calc. for C9H1602 (156.22) C 69.19, H 10.32%. IR: 3300–2700,
1720, 1660, 1040 cm–1. 1H NMR: 5.8 (ddt, J = 16.9, 10.2, 6.5
Hz, 1H), 4.99 (dm, J = 17.0, 1.5 Hz, 1H), 4.93 (dm, J = 10.2, 1.5
Hz, 1H), 2.35 (t, J = 7.5 Hz, 2H), 2.04 (m, 2H), 1.64 (m, 2H),
1.34 (m, 6H). MS (m/z): 55 (100), 41 (69), 69 (51), 60 (44), 96
(40), 68 (38), 73 (31), 138 (28), 67 (22), 82 (20).
A few ozonolytic methods for the synthesis of 9-hydrox-
ynonanoic acid are known from the literature (12,13). However,
they employ for the reduction of the intermediate ozonides a rel-
atively costly reducing agent, and the catalytic reduction of
ozonides with hydrogen also has disadvantages. We suggest a
new approach to the preparation of ω-hydroxynonanoic acid 1
which involves an electrochemical reduction of the intermediate
hydroperoxides of the methyl ricinoleate (Schemes 1 and 2).
Both ozonolysis and electrochemical reduction of the inter-
mediate products can be conveniently carried out in a methanol
or in an acetic acid solution (17). During the electroreduction
process in the presence of sodium bicarbonate (1.2 equiv) as a
support electrolyte, the ester group is hydrolyzed to give sodium
ω-hydroxynonanoate. This has an additional advantage from the
preparative point of view because, after methanol evaporation,
the enantiomerically pure (R)-1,3-nonanediol, the by-product,
can be isolated by extraction with ether. The diol is the major
component of the male rectal glandular secretion of Dacus tau
(18). It can also be employed to the two-step synthesis of the
sensorially important (R)-γ-decalactone (15). On the other hand,
by treatment of the aqueous solution with hydrochloric acid (pH
< 2), the 9-hydroxynonanoic acid could be extracted with ether
and purified by recrystallization from ether to give white crys-
tals, m.p. 51–52°C.
Another method which we have tried for the preparation of
9-hydroxynonanoic acid was castor oil ozonolysis in methanol
followed by sodium borohydride reduction of the intermediate
hydroperoxides (Scheme 2). Although the method is known,
commercial castor oil was employed to that purpose for the first
time in the ozonolysis process. Use of commercial castor oil was
possible because of the high content of ricinoleic acid in castor
oil (up to 90%) and, contrary to other oils, the castor oil is solu-
Alkyl 8-nonenoates 5a–5d. To a solution of 8-nonenoic
acid 4 (0.02 mole) in toluene (10 mL), 0.5 mole of the appro-
priate alcohol and 0.2 g Amberlyst 15 were added, and the
mixture was refluxed for 5 h. Thereafter, the catalyst was fil-
TABLE 1
Boiling Points and Specral Data of Diesters 3a–3d
Compounda
B.p. (°C/mm Hg)
116–118/0.8
Yield (%)
55
1H NMR
GC–MS (m/z)
IR (cm–1)
3a
4.05 (t, J = 7.5 Hz), 3.67 (s),
2.31 (t, J = 6.5 Hz), 2.05 (s),
1.64–1.59 (m), 1.35–1.29 (m)
43 (100), 55 (70), 74 (64), 97 (42),
41 (31), 69 (28), 87 (25), 96 (23),
59 (21), 138 (17)
1760–1720, 1440,
1240, 1170, 1035,
760
3b
3c
3d
146–149/1.5
120–121/03
125–128/0.3
50
79
71
4.12 (q, J = 7.5 Hz), 4.05 (t, J = 7 Hz),
2.30 (t, J = 7.5 Hz), 2.05 (s),
1.61 (m), 1.23 (m)
43 (100), 55 (73), 97 (48), 88 (44),
69 (34), 41 (31), 61 (27), 96 (20),
70 (19), 101 (18)
1760–1725, 1470,
1235, 1180, 1040,
760
4.05 (m), 2.30 (t, J = 7.5 Hz),
2.04 (s), 1.62 (m), 1.31 (m),
0.94 (t, J = 7.5 Hz)
43 (100), 55 (96), 61 (75), 97 (69), 1760–1725, 1470,
69 (60), 41 (53), 157 (46), 60 (36), 1230, 1170, 1040,
84 (26), 96 (26)
760
4.05 (m), 2.29 (t, J = 7.5 Hz),
2.04 (s), 1.63 (m), 1.31 (m),
0.93 (t, J = 7 Hz)
43 (100), 55 (77), 56 (70), 97 (49), 1750–1730, 1465,
41 (48), 69 (47), 157 (40), 57 (32), 1230, 1175, 1040,
139 (20), 84 (18)
760
a(3a) methyl 9-acetoxynononoate, (3b) ethyl 9-acetxynonanoate, (3c) n-propyl 9-acetxynonanoate, (3d) n-butyl 9-acetxynonanoate. Abbreviations:
NMR, nuclear magnetic resonance; GC–MS, gas chromatography–mass spectroscopy; IR, infrared.
JAOCS, Vol. 76, no. 7 (1999)

814
J. KULA ET AL.
TABLE 2
Boiling Points, Purities, and Spectral Data of Unsaturated Esters 5a–5d
B.p.
(°C/mm Hg)
Yield
(%)
Purity
(%)
Compounda
5a
1H NMR
GC–MS (m/z)
IR (cm–1)a
96/14
88/1
61
74
60
95
96
82
5.80 (ddt, J = 16.9, 10.2, 6.5 Hz), 5.0
(dm, J = 17 Hz), 4.91 (dm, J = 10.2 Hz),
3.66 (s), 2.3 (t, J = 7.5 Hz), 2.05 (m),
1.64 (m), 1.3 (m)
88 (100), 55 (85), 69 (60), 41
(59), 96 (55), 138 (45), 101
(36), 60 (34), 61 (28), 84 (21)
3100, 1755,
1655, 1210,
1180, 920
5b
5c
5.7 (ddt, J = 16.7, 10.3, 7.5 Hz), 4.92
(dm, J = 16.8), 4.86 (dm, J = 10.3 Hz),
4.05 (q, J = 7.5 Hz), 2.3 (m), 1.96 (m),
1.55 (m), 125 (m), 1.18 (t, J = 7 Hz)
88 (100), 55 (88), 41 (85), 69
(70), 138 (51), 96 (49), 60 (43),
101 (40), 61 (37), 72 (31)
3100, 1740,
1645, 1255,
1170, 910, 735
77–81/0.3
5.79 (ddt, J = 16.7, 10.3, 6.5 Hz), 4.98
(dm, J = 16.7 Hz), 4.93 (dm, J = 10.3 Hz), (82), 61 (76), 96 (73), 60 (62),
4.07 (t, J = 7 Hz), 2.29 (t, J = 7.5 Hz),
2.03 (m), 1.63 (m), 1.35 (m), 0.93
(t, J = 7.5 Hz)
55 (100), 41 (92), 69 (88), 43
3100, 1735,
1645, 1250,
1170, 910, 760
73 (43), 121 (42), 97 (33)
5d
78–82/0.1
56
77
5.8 (ddt, J =16.9, 10.4, 6.2 Hz), 5.02
(dm, J = 17 Hz), 4.92 (dm, J = 10.4 Hz),
4.03 (t, J = 7 Hz), 2.33 (m), 2.03 (m),
1.65 (m), 1.36 (m), 0.92 (t, J = 7.5 Hz)
55 (100), 41 (98), 56 (84), 69
(66), 57 (60), 96 (57), 60 (33),
73 (30), 121 (29), 97 (28)
3100, 1730,
1645, 1465,
1250, 1170,
1000, 970, 910
a(5a) Methyl 8-nonenoate, (5b) ethyl 8-nonenoate, (5c) n-propyl 8-nonenoate, (5d) n-butyl 8-nonenoate. For abbreviations see Table 1.
ble in methanol even at low temperature, e.g., at –15°C. Methyl convenient and has economic and environmental advantages.
ricinolate is also soluble in methanol, and is easily available by Electroreduction must be carried out until 8 F/mole is delivered
the chemical or enzymatic transesterification of castor oil. This because, if not, some aldehydic compounds still remain in the
is why castor oil is an excellent starting material in ozonolysis reaction product. The cost of the electricity for the electroreduc-
processes. As can be seen from the Experimental Procedures tion was at least 30 times lower as compared with sodium boro-
section, both methods give satisfactory results as far as the yield hydride consumption.
and purity of the hydroxy acid are concerned. However, an elec-
9-Hydroxynonanoic acid as a bifunctional compound may be
trochemical reduction of the intermediate is undoubtedly more of some interest for the preparation of diesters that, as such, are
O3/Pb-cathode
or O3/NaBH4
Methyl ricinoleate (castor oil)
1
SCHEME 2
2a–2d
1
3a–3d
5a–5d
4
a, R = Me; b, R = Et; c, R = n-Pr; d, R = n-Bu
SCHEME 3
JAOCS, Vol. 76, no. 7 (1999)

ω-HYDROXYNONANOIC ACID AND ITS DERIVATIVES
815
A
B
FIG. 1. Mass spectra of propyl-nonenoate (A) and butyl-nonenoate (B).
JAOCS, Vol. 76, no. 7 (1999)

816
J. KULA ET AL.
applied in cosmetics (19) or sometimes exhibit flavor properties 5a–5d to produce lower homologs of undecylenic acid esters
(20). Generally there is no literature concerning synthesis, spec- known from their attractive sensory properties (20). Spectral
tral characteristics, and olfactory properties of ω-ace- data and purities (GC) of the unsaturated esters are shown in
toxynonanoic acid esters. Now we show a series of diesters Table 2. The esterification was carried out in the presence of
3a–3d that were prepared from readily available starting mater- Amberlyst 15 as catalyst because the reaction catalyzed by
ial using the standard reaction sequence (Scheme 3). The sub- concentrated sulfuric acid resulted in formation of products
strate monoesters (methyl, ethyl, n-propyl, and n-butyl) of the in a yield of 42–60% containing scarcely 60–82% of the ex-
9-hydroxynonanoic acid (2a–2d) were prepared by direct es- pected esters. The structure of the esters 5a–5d was confirmed
1
terification of the hydroxy acid with the corresponding alco- by spectral data. H NMR spectra of all the esters gave an
hols. Their physical and spectral data coincided with those re- H-H olefinic coupling pattern that matched that characteristic
ported in the literature (1,12,13).
The boiling points and spectral data of the acetoxy esters the propyl and the butyl esters were only 82 and 77% pure,
3, supporting their structures, are shown in Table 1. respectively. GC analysis indicated that both products con-
Neither the hydroxyesters nor the acetoxyesters had an in- tained (13 and 18%) two additional compounds in a 2:1 ratio.
teresting odor. They are probably odorless when pure. The other two esters, 5a and 5b, contained only a negligible
When 9-hydroxynonanoic acid was purified by distillation amount of such an impurity and were not identified.
under reduced pressure, a small quantity of a lower boiling frac- GC–MS spectra of the accompanying compounds (Fig.
for 8-nonenoic acid. However, as can be seen from Table 2,
tion could sometimes be collected. We were intrigued by the 1A, 1B) show molecular peaks at 198 and 212 corresponding
substance and, after 1H NMR analysis, it appeared that the ma- to propyl and butyl nonenoate. The spectrum in Figure 1A
terial was mainly composed of 8-nonenoic acid 4. The mecha- contains a large signal m/z = 43 that is characteristic for the
nism of the unsaturated acid formation is likely of a dimer propyl fragment, and Figure 1B possesses a peak m/z = 57
(polymer) pyrolysis origin. Pyrolysis of esters is a known characteristic for the butyl fragment. A strong peak m/z = 138
method for the production of unsaturated compounds (21). On present in both spectra is a result of propanol and butanol
the other hand, hydroxy acids tend to dimerize (polymerize) elimination from the ester molecule. The major signal m/z =
under elevated temperature (22). Thus, the thought of combin- 55 is assumed to originate from a stable allylic cation,
+
ing the two reactions in a one-step process is attractive. How- CH3CH=CHCH2 , which can readily form from the unsatu-
ever, to make the process more effective high-temperature py- rated moiety of the ester. Interestingly, the fragmentation
rolysis was necessary. This could be easily reached by distilling pathways of the considered compounds coincide with the MS
9-hydroxynonanoic acid under atmospheric pressure in the data for cis and trans methyl 7-noneate (23). We concluded
presence of argon. Under conditions we tried, the yield of the therefore that the mass spectra in Figures 1A and 1B were of
pure (92%, GC) 8-nonanoic acid amounted to 28%, and the py- propyl and butyl 7-nonenoate, respectively. Apparently, the
rolysis reaction was difficult to control.
activity of the catalyst was sufficient to cause the double bond
A number of widely different methods have been described migration at an elevated temperature. All the unsaturated es-
for the preparation of terminal unsaturated carboxylic acids in- ters 5a–5d were found to have olfactory activity. For instance,
cluding ω-chloro- and ω-acetoxy-carboxylic acids as starting ethyl 8-nonenoate demonstrated a very strong and clean
materials, but the yields of the unsaturated acids were rather low pineapple aroma.
(21). We prepared 8-nonenoic acid 4 in a two-step conversion of
9-hydroxynonanoic acid 1 (Scheme 3). In the first step hydroxy
acid 1 was reacted with boric acid to give quantitatively ortho-
ACKNOWLEDGMENTS
borate ester which in the next step, underwent pyrolysis under This work was supported in part by research grant from the State
Committee for Scientific Research (3T09 B03012). The authors
thank Ewa Ulatowska-Wdowiak, Anna Tichek, and Teresa Majda
for the performance of the GC and GC–MS analyses.
atmospheric pressure. In this way 8-nonenoic acid 4 could be
obtained in 45% overall yield from 9-hydroxynonanoic acid. It
was 95% pure and its structure was confirmed by infrared, 1H
NMR, mass spectra, and elemental analysis. Thus, the olefinic
REFERENCES
proton at C-8 in the NMR spectrum of the unsaturated acid 4 ap-
peared at 5.80 ppm as ddt with J = 16.9, 10.2 and 6.5 Hz owing
to trans and cis coupling with C-9 olefinic protons. The olefinic
protons at C-9 at 4.99 and 4.93 ppm showed as a double doublet
with J = 17.0, 1.5, and 10.2, 1.5 Hz, respectively, clearly indi-
cating the terminal position of the double bond. It should be
noted that the NMR spectrum contained also a small, strange
triplet (J=6.5 Hz) at 0.88 ppm and a multiplet at 5.2 ppm dis-
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